1. Field of the Invention
The present invention relates to a video signal encoding method and apparatus, and more particularly to a method and apparatus for generating a bounding rectangle of a video object plane (VOP) for interlaced scan type video signals.
2. Description of the Prior Art
Video encoding schemes are classified into a frame-based encoding scheme in which pixels existing in the entire region of a rectangular frame are encoded, and an object-based encoding scheme in which only arbitrary shaped regions of a rectangular frame are encoded. For the frame-based encoding scheme, there are H.261 and H.263, which are vidio coding standards recommended by International Telecommunication Unionxe2x80x94Telecommunication Standardization Sector (ITU-T), and MPEG-1 and MPEG-2 standardized by Moving Picture Experts Group (MPEG) of International Standardization Organization/International Electrotechnical Commission Joint Technical Committee 1/Sub Committee 29/Working Group 11 (ISO/IEC JTC1/SC29/WG11). On the other hand, for the object-based encoding scheme, there is MPEG-4 standardized by MPEG of ISO/IEC JTC1/SC29/WG11.
In the object-based encoding scheme, typically, an input video is divided into a plurality of video object planes (VOPs) and processed on the basis of VOP. Each VOP is defined by a bounding rectangle surrounding an arbitrarily shaped object.
Generally, video encoding according to the object-based encoding scheme involves the steps of: extracting luminance shape information from an input video signal and forming VOPs using extracted luminance shape information; and encoding video information of every VOP. The step of encoding video information of every VOP consists of 3 substeps. The 1st substep is encoding shape information of every VOP, based on the extracted luminance shape information. The 2nd substep is subsampling the extracted luminance shape information, thereby extracting chrominance shape information from the extracted luminance shape information, padding a chrominance signal, based on the extracted chrominance shape information, and encoding chrominance of every VOP. And, the 3rd substep is padding a luminance signal, based on the extracted luminance shape information, and encoding luminance of every VOP.
In such a video encoding scheme, a conservative chrominance shape subsampling method has conventionally been used as the subsampling method for extracting chrominance shape information from the extracted luminance shape information. This method is a subsampling method in which chrominance shape information is extracted using four adjacent luminance shape informations defining a rectangle. An example of such a conservative chrominance shape subsampling method is disclosed in Europe Patent No. EP806871 issued to Dae Woo Electronics Co., Ltd. (titled: METHOD AND APPARATUS FOR GENERATING CHROMINANCE SHAPE INFORMATION OF A VIDEO OBJECT PLANE IN A VIDEO SIGNAL).
Meanwhile, MPEG-4 not only supports coding of progressive scan type video signals but also supports coding of interlaced scan type video signals for an application to broadcasting fields, as MPEG-2. In this regard, input video signals for coding and subsampling may be frame type video signals according to a progressive scan scheme in which all signals of one frame of video are simultaneously acquired while being scanned within the same period of time, as shown in FIG. 1a, or may be field type video signals according to an interlaced scan scheme in which signals of one frame of video are sorted into those of a top field and those of a bottom field which are different from each other in terms of the video signal acquisition time, as shown in FIG. 1b. 
The above mentioned conventional conservative chrominance shape subsampling method is associated with coding of progressive scan type video signals. Although this method does not involve any problems in association with video signals of the progressive scan scheme, it involves a color bleeding problem in association with video signals of the interlaced scan scheme in which luminance shape information is extracted in an interlaced manner for different fields. This will be described in detail, in conjunction with FIG. 2. In FIG. 2 and following figures, xe2x80x9cXxe2x80x9d represents luminance shape information, xe2x80x9c xe2x80x9d represents chrominance shape information, xe2x80x9c xe2x80x9d represents information belonging to a top field, and xe2x80x9c xe2x80x9d represents information belonging to a bottom field. Where subsampling of interlaced scan type video signals is carried out for every rectangular subsampling block defined by four adjacent luminance shape informations, based on the luminance shape information, in accordance with the conventional conservative chrominance shape subsampling method, a color bleeding problem occurs because both of chrominance shape information existing in the top field and chrominance shape information existing in the bottom field should be extracted, based on luminance shape information existing in the top field and luminance shape information existing in the bottom field.
In order to solve such a problem, a field-based chrominance shape subsampling method has been proposed which is a subsampling method for interlaced scan type video signals and disclosed in the pending Korean Patent Application No. 98-7374 filed on Mar. 5, 1998 by the applicant. This method is a subsampling method in which chrominance shape information is extracted, based on four adjacent luminance shape informations existing in the same field and defining a rectangle.
However, even though the field-based chrominance shape subsampling method disclosed in the Korean Patent Application No. 98-7374 is applied, extraction of chrominance shape information may be erroneously carried out in such a fashion that, as shown in FIG. 3, chrominance shape information i existing in the bottom field is extracted based on luminance shape information c, d, g, and h existing in the top field while chrominance shape information existing in the top field is extracted based on luminance shape information existing in the bottom field, if bounding rectangles defining a subsampling range of chrominance shape information are erroneously set.
Therefore, the present invention has been made in view of the above mentioned problem involved in the prior art, and an object of the invention is to provide a method and apparatus for generating a bounding rectangle of a VOP for interlaced scan type video signals, which method and apparatus are capable of accurately generating a bounding rectangle, thereby preventing a color bleeding from occurring in the field-based chrominance shape subsampling.
In accordance with one aspect, the present invention provides a method for generating a bounding rectangle of a video object plane for interlaced scan type video signals comprising: bounding rectangle formation step of forming a bounding rectangle of a video object plane in such a fashion that said bounding rectangle contains an object therein, based on input luminance shape information; optimum setting determination step of determining whether or not a spatial reference point of said bounding rectangle is positioned on an x-y coordinate of coordinate value (2m, 4n) (m and n=0, 1, 2, 3, 4, . . . ); and bounding rectangle adjustment step of varying said bounding rectangle, when it is determined at said optimum setting determination step that said spatial reference point of said bounding rectangle is not positioned on said x-y coordinate of coordinate value (2m, 4n), in such a fashion that said spatial reference point of said bounding rectangle is positioned on said x-y coordinate of coordinate value (2m, 4n).
In accordance with another aspect, the present invention provides an apparatus for generating a bounding rectangle of a video object plane for interlaced scan type video signals comprising: a bounding rectangle formation unit for forming a bounding rectangle of a video object plane in such a fashion that said bounding rectangle contains an object therein, based on input luminance shape information; an optimum setting determination unit for determining whether or not a spatial reference point of said bounding rectangle is positioned on an x-y coordinate of coordinate value (2m, 4n) (m and n=0, 1, 2, 3, 4, . . . ); and a bounding rectangle adjustment unit for varying said bounding rectangle, when said spatial reference point of said bounding rectangle is not positioned on said x-y coordinate of coordinate value (2m, 4n), in such a fashion that said spatial reference point of said bounding rectangle is positioned on said x-y coordinate of coordinate value (2m, 4n).